1. Field of the Invention
This invention is related to apparatus for providing lightning protection to a digital subscriber loop interface.
2. Description of the Prior Art
The digital subscriber loop (DSL) interface is difficult to protect against lightning because it is a low impedance circuit to match the low impedance of the DSL. The DSL impedance is about 100 ohms over the frequency band, 125 kHz to 250 kHz, used to transmit the digital signals. The 100 ohm impedance is low compared to the 600 to 900 ohm impedance of the line at voice frequencies. Thus, the two battery feed series resistors used to protect the semiconductor subscriber line interface circuit (SLIC), which are each selected to be one-half of the line impedance, can not be used with the DSL interface because they would increase the circuit impedance far above the nominal impedance. This in turn would cause high attenuation of the transmitted and received signals, and would also cause echos due to impedance mismatch. This invention is intended to overcome these problems and to protect the DSL interface.